The present invention extends the teachings of my co-pending application for patent, Ser. No. 947,344, filed Oct. 2, 1978, now U.S. Pat. No. 4,183,405 which is incorporated herein by reference.
Petroleum reservoirs commonly are located underground in porous, permeable rock strata. Due to tectonic activity the petroleum reservoir frequently is inclined with respect to the horizontal. The petroleum itself is trapped in the pore space of the host rock, generally having arrived at its trapped position by displacing remnants of an ancient ocean. Under these conditions the up dip limits of the petroleum reservoir are defined by a blockage such as a permeability pinch-out or a fault. The down dip limit of the reservoir generally is the displaced salt water and such limit is commonly called the oil/water contact. The oil/water contact generally is located in a transition zone of the reservoir, such transition zone being positioned roughly parallel to the horizontal. The up dip limit of the reservoir often is quite irregular and its exact location uncertain, such location being known only to the extent that it lies between a producing well and a dry hole. The up dip limit may lie near the producing well, near the dry hole or some point in between.
Generally each oil producing province in the United States has well spacing regulations that establish drilling patterns for the petroleum reservoir. Such spacing generally is uniform for the entire reservoir, with one reservoir assigned a 40 acre spacing, another a 160 acre spacing, and the like. For a 40 acre spacing each well drilled would be one fourth of a mile from adjacent wells, and for 160 acre spacing the wells would be a half mile apart.
Looking now to development drilling of a newly discovered dipping oil reservoir in which the state regulatory agency has set for 160 acre spacing, step out drilling continues until the maximum limits of the reservoir are substantially defined. In the usual case, step out drilling results in several dry holes, the wells being designated dry in the sense that they are incapable of producing petroleum in commercial quantities. Step out drilling in a down dip direction ultimately will result in a well being drilled below the oil/water contact, a dry hole capable of producing brine. Step out drilling in both strike directions ultimately will result in at least two dry holes which are located beyond permeability pinch-outs or faults that define the lateral extent of the reservoir. Step out drilling in an up dip direction from a series of producing wells located on strike often results in some wells becoming producers and others being dry holes, presenting the operator with somewhat of a paradox.
A prudent operator will not continue to drill a well pattern that appears to be located below the oil/water contact. Even if some of the wells might be producers, their life would be short as the oil/water contact moves up dip during continuing production. Such oil as might have been produced under these conditions generally can be poroduced at a later time through existing production wells positioned up dip. The operator also will refrain from drilling additional wells suspected to be beyond the lateral extent of the reservoir. The row of wells up dip, some producers and some dry holes, are of particular concern when the reservoir is devoid of a natural gas cap. Each dry hole in the row of wells up dip provides positive information about the up dip limits of the reservoir: the limit is somewhat between the dry hole and the corresponding producer down dip. The producer wells in the row of wells up dip are obviously in the reservoir, with the limit of the reservoir at some location further up dip. The operator then is faced with the decision of whether or not to offset a producing well with a new well drilled at the next up dip location, recognizing the high risk of drilling a dry hole. It would be quite helpful to the operator if there were a method to measure the up dip extent of the reservoir without incurring the expense of drilling additional dry holes. It is one object of the present invention to teach such methods.
At each producer well located near the dip extremity of the reservoir, there is an unknown amount of petroleum located between the producer and the up dip limit of the reservoir. This unknown amount of petroleum will remain locked in place when the water drive advances up dip to the well location and causes the producer well to water out and thus no longer be a producer of petroleum in commercial quantities. This petroleum locked in place in the attic could be produced with a newly drilled well positioned off pattern near the upper limit of the reservoir. Siting such off pattern well is a risky undertaking which requires special approval from the state regulatory agency, approval that may or may not be forthcoming. The petroleum locked in place, sometimes called attic oil, often involves 100,000 barrels or more, and thus its recovery generally is a worthwhile effort.
During recent years many schemes have been tried in the quest for recovery of attic oil. One method, previously mentioned, is to drill off pattern wells. This method requires precision well siting, and even then some oil will remain in the attic. A less risky method involves creating an artificial gas cap that converts a portion of the reservoir to a combination water drive/ gas drive. In the early stages of trying this method, generally there was an abundant nearby supply of low value natural gas. The natural gas was injected into one or more up dip production wells at such pressure needed to overcome the water drive pressure. Upon continuing injection of natural gas in this manner, attic oil would take natural gas into solution until becoming saturated, then surplus natural gas would form an ever increasing gas cap. Eventually the gas cap would expand to such an extent that the attic would be, in effect, composed of a gas cap. Under the influence of a gas drive, recoverable attic oil would be produced by pressure relief into a down dip production well. This method relies on the availability of natural gas and has the disadvantage of leaving a substantial amount of natural gas in the attic.
In recent times the value of natural gas has increased dramatically and regulatory agencies have established priorities for its use. Currently there are many instances where natural gas, although produced in the area, is not available for use in creating an artificial gas cap in a petroleum reservoir. There are, however, many other gases that are suitable for taking into solution in attic petroleum to form an artificial gas cap and a gas drive to the reservoir. Some promising candidates include carbon monoxide, carbon dioxide and nitrogen, all of which are readily available in the exhaust stream generated by the combustion of hydrocarbon fuels. There have been several attic oil production projects that have used products of combustion to create the desired artificial gas cap.
Products of combustion are particularly desirable when they are available in copious quantities as a by-product of another operation, such as the exhaust from internal combustion engines that drive a battery of compressors. These gases are normally vented to the atmosphere together with the water vapor that also is a product of combustion. These gases may be diverted to attic oil production, although they are not readily usable without further processing. Since the gases must be compressed to pressures exceeding that of the underground attic oil pressure, it is necessary that the water vapor be removed prior to compression. Also, during the combustion process in internal combustion engines some of the nitrogen from the intake air combines with oxygen to form nitrous oxide. With only 400 parts per million of nitrous oxide, a million standard cubic feet of inert exhaust gas can contain almost 50 pounds of nitric acid, an ingredient that must be substantially removed prior to compensation. Removal of water vapor is a relatively inexpensive undertaking, while removal of nitrous oxide is generally considerably more expensive. Nitrous oxide does not readily form at moderate temperatures of combustion, but is almost always found in the higher temperatures inherent in an internal combustion engine. Generating exhaust gases at moderate temperatures of combustion and thus avoiding formation of nitrous oxide is highly desirable as will be described hereinafter.
As is well known in the art a gas drive for petroleum performs better when the gases used are readily soluble in petroleum. The solubility capability of a medium grade crude oil at a reservoir pressure approximating 2000 psi at a temperature of 120.degree. F. typically is, in standard cubic feet per barrel:
TABLE 1 ______________________________________ carbon dioxide 1200 natural gas 660 carbon monoxide 83 nitrogen 70 hydrogen 68 ______________________________________
Since the solubility of one gas is substantially unaffected by the presence of another, a considerable amount of crude dilution can be effected by injecting the suite of gases listed in Table 1. As pointed out previously, natural gas may not be available for this purpose. Carbon dioxide, carbon monoxide and nitrogen can be made readily available at many sites as a product of combustion. Hydrogen, not normally a product of combustion derived from internal combustion engines, can be made available in a type of combustion involving coal as the fuel.
Coal deposits are common at sites overlying petroleum reservoirs, and thus there are many cases where combustion of coal can be used to generate gases useful in the production of attic oil. Coal can be gasified in an aboveground generator such as the well known Lurgi system or it may be generated from coal in situ. In both cases temperatures are in the moderate range, generally eliminating the formation of the undesirable nitrous oxide when combustion is attained using air as the oxidixer. The producer gas generated, on a volumetric dry basis, typically is:
TABLE 2 ______________________________________ Aboveground In Situ ______________________________________ hydrogen 10.5 17.2 carbon monoxide 22.0 14.7 carbon dioxide 5.7 12.4 nitrogen 58.8 51.0 methane plus 3.0 4.6 ______________________________________
All of the gases listed in Table 2 are readily soluble in attic oil. This suite of gases can find a useful purpose in establishing a gas drive in attic oil and, upon continuing injection, in the creation of an artificial gas cap.
For the purpose of measuring the size of the attic, the diffusion characteristics of gases listed in Table 2 provide a useful tool. Diffusion properties of the suite of gases where the diffusion rate of carbon dioxide is taken at unity may be expressed as:
TABLE 3 ______________________________________ carbon dioxide 1.0 carbon monoxide 1.6 nitrogen 1.6 methane 1.5 hydrogen 22.0 ______________________________________
Hydrogen, with its relatively low solubility capability, can be expected to move relatively rapidly through the petroleum reservoir with continued injection of the suite of gases. It is this special characteristic of hydrogen that is of interest in measuring the approximate size of the attic. It will be appreciated that this invention is not limited by an theory of operation, but any theory that has been advanced is merely to facilitate disclosure of the invention.
It will be appreciated that all gases listed in Tables 1, 2 and 3 will, upon injection into the petroleum reservoir, first become partially dissolved into the petroleum, then begin to migrate to the highest permeable point in the reservoir. Gases with low diffusion rates will tend to supersaturate the up dip petroleum in the immediate vicinity of the injection well, while hydrogen will tend to diffuse in an up dip direction before full saturation is attained. Upon continued injection of the suite of gases, a gas cap will form beginning at the up dip limits of the reservoir and expanding down dip. Thus production wells in the updip portion of the reservoir can be produced under the influence of the gas drive created by the artificial gas cap, and after a period of time will cease to produce oil when they become engulfed by the expanding gas cap. Likewise the production wells in the down dip portion of the reservoir can be produced under the influence of the water drive until they become engulfed by the advancing water.